Conventionally, granules of synthetic resin are dried, or those of petrochemical resin of crystalline synthetic resin are crystallized, in manner such that usually a sealed processing tank is used and the granules charged therein are supplied with hot air from below.
In this case, when the temperature of the material becomes higher than a predetermined level, additional granules are softened and mutually fused to make difficult uniform hot-air-drying, or are oxidized at the surface, whereby they are not usable as molding material. Hence, the granules must be dried by hot air at a lower temperature than the predetermined level, thereby creating the problem in that additional time is required to that extent for drying them.
The present invention relates to granules including a water content which are heated from the interior by microwaves, so that such heating is convenient for dispersing the water content in the granules onto the surfaces thereof. The technique includes a granule drying and crystallizing apparatus which can dry or crystallize the granules in a short time by using the microwaves. The above idea is the subject of Japanese Patent Application No. Sho 62-314920 of the present invention.
Such internal heating is very efficient, but has a problem in that variation is caused in the heating of granules in the processing tank. The problem is remedied at one side in manner such that the granules are agitated by rotation of vanes provided in the processing tank. The use of such usual agitating vanes, however, results in a non-uniform residence time of granules in the processing tank, in other words, the time of microwave-heating the granules while staying in the tank, thereby creating a problem in that variation in the heating still results.
On the other hand, the heating by the microwaves is efficient to quickly reach the temperature of drying the granules, whereby they are heated prior to being dried by vaporizing the water content. As a result, the radiation of the microwaves must often be interrupted so as not to fuse the granules due to an excessive high temperature during the drying step. Accordingly, a problem also is created in that an expensive microwave apparatus is idle during part of the process without a full operation, which is very uneconomical.